1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type for forming an image, such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
In the past, there have been proposed various image forming apparatuses which include a plurality of image forming portions and in which different color toner images are formed in the respective image forming portions and are successively transferred onto a single recording material in a superimposed fashion to thereby form a color image. In such a circumstance, image forming apparatuses of electrophotographic type utilizing an endless recording material bearing member have been used for high speed recording.
Explaining an example of the image forming apparatus with reference of FIG. 11, first, second, third and fourth image forming portions Pa, Pb, Pc and Pd are disposed side by side within a main body of the apparatus, and different color toner images are formed through processes (latent image formation, development and transferring).
The image forming portions Pa, Pb, Pc and Pd include exclusive image bearing member (in this example, electrophotographic photosensitive drums 3a, 3b, 3c and 3d) and different color toner images are formed on the respective photosensitive drums 3a to 3d. A transfer belt 200 is disposed adjacent to the photosensitive drums 3a, 3b, 3c and 3d, so that the different color toner images formed on the respective photosensitive drums 3a, 3b, 3c and 3d are successively transferred onto a recording material P born on the transfer belt 200 in a superimposed fashion. At the same time, the recording material P is electrostatically absorbed onto the transfer belt 200. Further, the recording material P to which the different color toner images were transferred is separated from the transfer belt 200 at a separation portion, and the separated recording material is sent to a fixing portion 9, where the toner image are fixed to the recording material. Thereafter, the recording material is discharged out of the apparatus as a recorded matter.
When image formation is performed under a low humidity environment, in some cases, image distortion such as a bird's foot print image is generated at the separation portion. That is to say, if a gap is generated between the transfer belt 200 and the recording material P at the separation portion to cause a peel discharge in the gap, the toner transferred to the recording material is scattered, thereby distorting the image.
In order to prevent the image distortion due to the discharge, a separation charge device 32 is used. A corona charger (charger of non-contact type) is used as the separation charge device 32 and is disposed in an opposed relation to a drive roller 210. AC voltage is applied to the separation charge device 32, thereby removing electricity from the recording material P and the transfer belt 200. By reducing potential of the recording material P and the transfer belt 200 by means of the separation charge device 32 to reduce intensity of an electric field in the gap, generation of the peel discharge is suppressed. By the removal of electricity, an electrostatic absorbing force of the recording material P to the transfer belt 200 is reduced, so that separation ability of the recording material P from the transfer belt 200 is stabilized and the peel discharge is relieved.
Further, since the drive roller 210 has low resistance, it is also used as a counter electrode for the separation charge device 32, and a conductive member having high coefficient of friction such as rubber is used for covering an outer surface of the drive roller 210.
However, when the scattered toner is adhered to the electrode of the corona charger, the performance of the charger is frequently worsened. In such a case, since the charger (performance of which was worsened) to which the toner was adhered must be exchanged, or a cleaner for cleaning the electrode of the charger must be provided, the entire apparatus is made more expensive.